It has been a challenging summer, but we are on track!
The heat has been a challenge with operating the Alpha Electros this summer, but we have come up with some creative solutions that have allowed us to continue accruing hours toward our goal of 100 hours on one aircraft to support our petition for exemption with the FAA. We currently have over 70 hours across all four aircraft and over 50 hours on N197AM that was chosen to be the plane for the endurance testing for the petition.
The heat issues began to show themselves when ambient temperatures in Fresno started to get above 90 degrees F. Our initial concern was the battery temperature, but we also found a limiting factor was the temperature of the power controller, as Pipistrel refers to it. It is the device than converts the electric power from the batteries and provides it to the electric motor. It is liquid cooled using a small radiator mounted just below the motor. The same radiator provides cooling for the motor, but the temperature limits for operation of the motor are higher than the power controller. We found it was possible to have the power controller overheat quickly doing touch and goes on days when ambient temperature was over 90 degrees F. The solution was to use less power on take-off and for climbing during cruise operations. We found early on that it is possible to take-off at 40 kW vs. 65+kW at full gross weight and still climb at about 500 ft. per minute. We also learned the planes would climb slowly at 23 kW during cruise vs. 40 kW. By running less amperage through the power controller, the temperatures stayed lower, even on high ambient temperature days. We have tested this procedure over many days at 100+ degrees F and it works, so it has become our standard operating procedure on high ambient temperature days.
Another issue that arose during these hot days was the battery temperatures would exceed 40 C when charged in the hot hangar. Once the batteries were hot, they took over-night to cool back down and that limited the number of flights you could get to about 1-2 per day per aircraft. Clearly that was not going to work. The solution was to air condition the batteries during charging. I was able to find a small 6,100 btu rated spot air conditioner designed for cooling computer servers on Amazon for about $500. I had some clear Lexan windows cut to fit the openings in the battery compartments with a hole for a flange to attach dryer ducting and then used a “plastic tee” to split the air flow from the A/C unit to cool both batteries packs at the same time. The A/C unit puts out about cooled air at about 17 C that then circulates around the battery packs during charging. If the charging is done right after a flight, the automatic cooling fans on each pack turn on to draw the cooled air in even faster. The end result is we are able to cool the battery packs during charging so they are less than ambient temperature by the time the charging cycle is complete. This allows for another flight immediately after charging. We are now about to get about 4 flights per day per aircraft. Below is a photo of the A/C unit in action:
The A/C system is not optimized and we are going to work on building better adapter fittings for the battery compartment openings to have better air seals, but it works for now and we will refine it this winter when we won’t need the cooling.
Finally, we have been steadily trying to get more time aloft with the planes as we continue our testing and validation work. This past week due to some collaboration with other pilots interested in our work, we were able to get 1 hour of flight time on a charge and still land over 20% State of Charge. The procedure involves using the 40 kW take-off procedure, but when leveling off in cruise, we are trimming the planes for best L/D speed of 64 kts and setting the power for 14 kW with a single pilot. We have flown the aircraft for over 59 nm in this mode and achieved an impressive 3.62 nm/kWh fuel economy. When you convert that to statue miles per kWh to compare with an electric car, it equals 4.16 miles per kWh which is more than my Chevy Volt!
That is all for now, but if you would like to come and see the aircraft in operation, we will be doing some flight demonstrations at Fresno Chandler Executive Airport on September 29, 2018 as part of the 2018 “Remember When Fly-in and Car Show” sponsored by the Central Valley Aviation Association. Gates open at 9 am for the public and pilots are invited to bring their aircraft for display. Display aircraft should plan to arrive by 8:30 am at the latest. Event ends at 4 pm.